Update Dockerfile.arm (#2150)

added NLTK lib  install

### What problem does this PR solve?


### Type of change
- [x] Performance Improvement

Dockerfile.arm

 WORKDIR /ragflow
 
 COPY requirements_arm.txt /ragflow/requirements.txt
+
+
+RUN pip install nltk --default-timeout=10000
+
 RUN pip install -i https://mirrors.aliyun.com/pypi/simple/ --default-timeout=1000 -r requirements.txt &&\
 python -c "import nltk;nltk.download('punkt');nltk.download('wordnet')"
 
 ADD docker/.env ./
 RUN chmod +x ./entrypoint.sh
 
-ENTRYPOINT ["./entrypoint.sh"]
+ENTRYPOINT ["./entrypoint.sh"]